Present compressors can be classified as follows: 1) piston (or reciprocating) compressors; 2) sliding blade compressors; 3) lobe compressors and 4) turbine (or radial flow) compressors.
Piston compressors. This type consists essentially of one or more pistons that run the interior of a hollow cylinder or sleeve in a reciprocating movement. Their operation is similar to that of a hypodermic syringe. These compressors are suitable where high pressures are required. Their theoretical maximum yield is 65%, and they can function up to ten thousand hours without requiring a general adjustment.
Sliding blade compressors. This type consists essentially of a hollow cylinder inside which another cylinder, containing several sliding blades placed radially, turns. Because the inside cylinder is disposed eccentrically with respect to the outside cylinder, the centrifugal force urges the sliding blades to describe an ellipse. The blades, in passing, compress the air, since the distance between the interior and the exterior cylinders diminishes. These compressors are suitable where large volumes of compressed air are required, at a relatively low pressure (6 or 7 atmospheres). These machines can function up to fifty thousand hours without needing major adjustments, and their maximum theoretical yield is 90%.
Lobe compressors. This type consists essentially of two or more rotors provided with synchronized lobes that turn inside a hollow cylinder. These compressors are suitable where large amounts of air are required at low pressures (maximum two atmospheres). Their yield is less than that of sliding blade compressors, but greater than that of piston compressors. Due to the eccentricity of the axes and the need for perfect synchronization between them, the mechanism easily loses adjustment, and therefore must be examined and adjusted frequently. One variation, known as a hydraulic piston type compressor, comprises a compressor piston (which is actually a lobe) comprising a bag full of oil, and is provided with blades. Upon turning, it follows the contours of a hollow cylinder whose interior is slightly oval. Because this compressor is not very reliable, and requires frequent maintenance, its use is highly restricted.
Turbine compressor. This type consists essentially of revolving elements (buckets) disposed in different radial positions along a single shaft. The shaft turns inside a long hollow cylinder, which has an inlet opening and an outlet opening. These compressors produce large volumes of air, but at a relatively low pressure (less than two atmospheres). The yield of this compressor is approximately 90%, and it can function up to sixty thousand hours without maintenance.
Revolving piston compressor. Because it is a revolving machine, it does not have appreciable kinetic losses. Therefore, its maximum theoretical yield is 90%. Its manufacturing cost is approximately one-third that of the sliding blade compressor, and approximately one-half that of the turbine compressor. This machine can reach pressures almost as high as those obtained with piston compressors, but with a higher yield. Its manufacturing cost is comparable to that of a reciprocating compressor having a similar volume flow rate capacity. In the revolving piston compressor, the entire cycle of the piston involves compression, as compared to traditional piston compressors, where one stroke is intake and the other compression.
As can be seen, this new revolving piston compressor has the advantages of both reciprocating compressors (high pressures and ease of construction) and revolving compressors (high yield, little maintenance and long duration). In addition, because the kinetic losses do not increase with the speed of the piston (as occurs with reciprocating compressors), the number of R.P.M. is not restricted, and, therefore a relatively small size machine can compress large volumes of air.